Ferromagnetic metal powders mainly composed of Fe and Co are used in the various fields of magnetic recording media, magnetic fluids and the like in which magnetism is utilized, because of their excellent magnetic characteristics.
In particular, with respect to magnetic recording media having a non-magnetic support and a magnetic layer formed thereon and mainly composed of a ferromagnetic powder and a binder resin, the ferromagnetic metal powder is used as the ferromagnetic powder to improve the recording density.
Such magnetic recording media come in practice as audio tapes, video tapes, floppy disks and the like.
The above ferromagnetic metal powders have the problem that they easily react with oxygen and water due to their chemical instability and therefore gradually deteriorate in their magnetic characteristics if placed in an air environment, particularly under the circumstances of high humidity.
In order to improve the chemical stability of the ferromagnetic metal powders, various methods have previously been proposed.
For example, there have been proposed a method of forming layers of protective oxides on the surfaces of the particles of ferromagnetic metal powders as described in JP-A-55-125205 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a method of forming protective layers of various organic compounds on the surfaces of the particles of ferromagnetic metal powders as described in JP-A-46-5057, and a method for densifying the structure of the layers of oxides by heat treatment to enhance the protective effect thereof of the layers of the oxides formed on the surfaces of the particles of ferromagnetic metal powders, as described in JP-A-59-273711.
In the above methods, the layers of the oxides are formed on the surfaces of the particles of the metal powders by contact with oxygen under relatively mild conditions, and such methods are referred to as so-called "moderate oxidation".
However, all of the prior-art "moderate oxidation" methods have disadvantages of some kind, so that the satisfactory chemical stability of the ferromagnetic metal powders can not be obtained.
Namely, the so-called "moderate oxidation" methods of forming the layers of the oxides on the surfaces of the particles of the ferromagnetic metal powders have the disadvantage that oxygen atoms gradually diffuse into the particles with time to cause the magnetic characteristics of the ferromagnetic metal powders to deteriorate.
In the methods of forming the protective layers of the organic compounds such as silicones and resins, it is necessary to coat the surface of the particles with relatively large amounts of organic compounds to obtain the protective layers enough to prevent oxygen and water from permeation. Accordingly, there is the problem that the saturation magnetization (.sigma.s) of the ferromagnetic metal powders is unavoidably reduced.
According to the methods of densifying the above layers of the oxides by heat treatment, if the heat-treating time is too long, oxygen diffuses into the particles and the oxide coatings become thick, which results in the reduced saturation magnetization. Conversely, if the heating time is too short, the oxide coating layers on the surface of the particles can not be sufficiently densified, and therefore the sufficient effect of the stabilization to oxidation can not be obtained. Moreover, it is difficult to control the conditions of the treating procedures and to obtain the stable characteristics.